Continuous dimming of lighting can be desirable for reasons of both energy efficiency and consumer preference. Dimming can change the atmosphere of the illuminated area, or, more practically, can adjust the electrical lighting in a space to compensate for variations in natural lighting.
The dimming of fluorescent lamps presents difficulties relating to the large voltage required to turn on the lamp through the initiation of an electrical arc in the tube. In particular, upon initial application of a normal operating voltage across the electrodes of a gas discharge lamp, insufficient numbers of free electrons exist in order to begin the cumulative ionization which carries the lamp current and maintains the discharge. Therefore, the electrodes must first be heated to thermionic emission temperature in order to produce the required free electrons. After conduction through the lamp has begun, it is not necessary for thermionic emission to continue.
Various methods have been employed to start gas discharge lamps, and, in particular, fluorescent lamps. One method involves the use of a starter switch operative to conduct the initially applied electrical current across the electrodes at either end of a fluorescent lamp. Once the electrodes have been electrically heated to thermionic emission temperature, the switch is opened, and the lamp is started. Alternatively, "starterless" circuits have been devised in which an AC current is applied to a standard ballast coupled to the lamp. A portion of the ballast output current is then used to heat the lamp electrodes to thermionic emission temperature.
Dimming devices for fluorescent lamps generally rely upon solid state components such as triacs to block portions of each half-cycle of the incoming voltage. By only allowing portions of each half-cycle to be conducted to the lamp, the amount of power delivered is thereby reduced. A problem with such dimming devices, when applied to gas discharge lamps, is that full power is needed to start the lamp because of the thermionic emission requirement discussed above. This means that a user must first apply full power to the lamp and then manually adjust the dimming device to the desired level. The dimming device cannot, therefore, be left at the desired setting since it must be readjusted every time the lamp is turned on. Also, when power outages occur, the lamps will not start automatically when power is returned if the dimming device is operative, again requiring manual intervention.
As noted above, dimming of incandescent lamps has been effected using phase control dimmers where energy to the load is controlled by varying the firing angle or "on" time of each half cycle of the A.C. supply power. However, while such dimmers when used in conjunction with standard fluorescent lamps coupled to standard lamp ballasts can satisfactorily control the energy to the high voltage portion of the lamps, at large firing angles the power supplied to lamp filaments may be insufficient to sustain illumination. Accordingly, full-range dimming of fluorescent lamps using phase control has been difficult to accomplish as a result of the minimum voltage required to adequately charge gases within such lamps.
It can thus be appreciated that it would be advantageous to have a method and apparatus for dimming fluorescent lamps over a full range of brightness. In addition, currently available fluorescent lamp dimmers require special ballasts for the lamp so that the filament is supplied its required power while either the voltage, current, or both supplied to the discharge potion of the lamp is varied to control brightness. A significant limitation of dimming systems using special ballasts is that the standard ballasts must be replaced with the special ballasts in order to dim the lamps involved. Hence, in order to avoid the expense involved in this replacement process it would be advantageous to provide a technique for modification of standard non-dimming ballasts to a configuration allowing for dimming of fluorescent lamps.
It would be additionally advantageous to provide a fluorescent lamp dimmer capable of being directly substituted for the two-wire on/off switch conventionally used in conjunction with fluorescent lamps. That is, dimming control circuits not requiring connection to a third "neutral wire" or the like could be straightforwardly substituted for existing two-wire lamp switches.
A dimming device which addresses the problem of starting gas discharge lamps is described in U.S. Pat. No. 4,950,963. The '963 device includes a circuit comprised of a diac, a triac and a timing capacitor for conducting an adjustable portion of an AC supply voltage to a lamp. In addition, the '963 device operates to conduct the entire AC waveform to the lamp for a preselected time upon initial application of power to the apparatus.
The device described above, however, is believed to be disadvantaged in at least several respects. For example, in order to convert the AC line voltage to a DC voltage for use by a timer circuit within the '963 device, a bridge rectifier is employed in combination with a resistive network. This resistive network dissipates heat, which could require mounting the device upon a heat sink or the like. In operation, the voltage impressed on a charging capacitor connected across a bridge circuit determines the voltage applied to the input of the diac. Unfortunately, the '963 device makes no provision for filming spurious electrical noise generated as a consequence of the delayed firing of the triae as determined by the time constant of the dimming control circuit. In addition, the '963 device requires connection to an AC neutral line of the fluorescent lamp fixture for which it is providing dimming control.